[mlir][test] Add emulator to the mlir-cpu-runner invocation

In https://reviews.llvm.org/D146917, MLIR's LIT configuration was
updated to allow us to use `mlir-cpu-runner` to run Arm SVE integration
tests. That update broke the following buildbot that doesn't support
SVE:

  https://lab.llvm.org/buildbot/#/builders/179/builds/6704

While that bot doesn't support SVE, it can run SVE tests under
emulation. This patch makes sure that whenever an Arm emulator is set
(via `RM_EMULATOR_EXECUTABLE` CMake variable), it is used to run both
`lli` _and_ `mlir-cpu-runner`.

I am sending this without a review as it's a rather trivial change and I
want to quickly fix the spurious bot failure.